An electronic copying apparatus is known in the prior art which is of the type wherein an electrostatic latent image is formed on a record medium and developed with a toner powder to produce a visible image on the record medium, which visible image is then transferred onto a transfer member, and wherein the record medium is subsequently neutralized by utilizing a corona discharge. The record medium is in the form of a drum or belt which undergoes an angular movement to cause a cyclic movement of its peripheral surface during which the copying process takes place.
An electrostatic latent image may be formed through a series of charging and exposure steps for a record medium which comprises a photoconductive, light sensitive member, and, in one case, may be formed by a selective charging of the surface of the record medium with a recording electrode such as a multistylus for a record medium which comprises a dielectric material.
A problem encountered in an electronic copying apparatus of the type described is dispersion of the toner powder into the copying apparatus and a marring of a neutralizer by the toner powder. The problem will be considered in more detail with reference to the drawings.
FIG. 1 illustrates an example of an electronic copying apparatus of the type described. A record medium 1 which comprises a photoconductive, light sensitive member is in the form of a drum which is rotatable in a direction indicated by an arrow. During a copying operation, the peripheral surface of the record medium 1 which rotates in the direction of the arrow is uniformly charged by a corona discharge from a corona charger 2, and is then irradiated with a light image which is formed by an exposure optical system 3. An electrostatic latent image thus formed on the record medium 1 is then developed with a toner powder by means of a developer unit 4, and the resulting visible image is transferred on a transfer member or paper 8 by means of a transfer unit 5. The visible image which is transferred to the transfer paper 8 is fixed by utilizing a fixing unit, not shown. The transfer paper 8 carrying the fixed visible image is delivered externally of the apparatus, thus providing a single copy.
After transfer of the visible image, the record medium 1 is neutralized by a corona discharge from a neutralizer 6, and any residual toner is removed by a cleaning unit 7.
However, a problem arises in conjunction with the use of the neutralizer 6 in that a dispersion of the toner powder into the apparatus and a marring of the neutralizer 6 by the toner powder may result.
Specifically, after the transfer of the visible image, a quantity of non-transferred toner remains on the peripheral surface of the record medium as it moves toward the location of the neutralizer 6. A majority of such non-transferred toner is distributed in those areas of the peripheral surface of the record medium which have not been brought into contact with the transfer member during the transfer step. An increased quantity of toner may remain in such areas where the transfer paper 8 has an area less than that of the visible image formed on the peripheral surface of the record medium or where the copying process is practised while leaving an original receptacle open which causes a dense deposition of the toner in border areas on the record medium so as to frame the visible image corresponding to the original. Even during a normal copying operation, there exists a certain potential, and hence a quantity of toner, around an image region. Referring to FIG. 2 which illustrates a transfer step of a visible image, the non-transferred toner which remains on the marginal area of a record medium which is not brought into contact with the transfer paper is strongly influenced by a corona discharge from the transfer unit 5, and is strongly charged to the same polarity as the corona discharge. In FIG. 2, character T represents toner.
Referring to FIG. 3, there is shown the neutralizer 6 which comprises a corona discharge wire 6a which is surrounded by a conductive shield plate 6b having a rectangular cross section with its one side removed. The shield plate 6b has opposite side plates 2a, 2b, the free end of which are located close to the record medium 1 with a spacing of a similar length. An a.c. corona discharge voltage is applied to the wire 6a, the a.c. corona discharge of which eliminates any residual charge on the toner T and any remaining charge on the record medium 1. However, as mentioned previously, the residual toner T which reaches the neutralizing region defined by the neutralizer 6 may be strongly charged to a negative polarity as shown, for example, during the transfer step, so that as it approaches the neutralizing region, the charge on the toner T induces an electric charge of the opposite polarity in the edge region of one of the side plates of the neutralizer 6 which is located at an advanced position, as viewed in the direction of movement of the record medium 1, namely, the side plate 2a, which is positioned opposite to the record medium 1, by an electrostatic induction.
The residual toner is charged to the same polarity as the peripheral surface of the record medium 1, and hence is subject to a force of repulsion from the peripheral surface, but is attracted to a potential of the opposite polarity on the conductive substrate of the record medium 1, whereby it is attached to the peripheral surface of the record medium 1 with a very reduced force and in an unstable manner. Then, if the charge is induced in the edge region of the advanced side plate 2a of the neutralizer 6, an electrostatic attraction occurs anew between the edge region and the toner T, whereby part of the toner T is transferred to the side plate 2a under the action of the Coulomb force and the gradient of electrical field. This causes a marring of the neutralizer 6 by the toner powder.
In addition, when the record medium is to be neutralized, the rotation thereof creates an air stream therearound, which causes a dispersion of part of the toner which is travelling from the peripheral surface of the record medium to the edge region into the various parts of the apparatus. The air stream and the centrifugalforce caused by the rotation of the record medium are a direct cause of dispersion of the toner T from the peripheral surface of the record medium.
The above description has been directed to the residual toner which remains after the completion of the transfer step, but the dispersion of the toner powder also occurs before the transfer step. Specifically, when an electrostatic latent image is being developed, a visible image is formed on the record medium by the use of the toner powder. Since some particles of the toner which forms the visible image are deposited on the record medium with a reduced electrostatic attraction or with a reduced adsorption effect due to van der Waals' force, when the air stream and the centrifugal force mentioned above are present, these toner particles readily loosen from the peripheral surface of the record medium to be dispersed into the apparatus.
A marring of the neutralizer 6 causes a change in the electrical parameters, which make it difficult to achieve a normal neutralizing effect. A dispersion of the toner into the apparatus causes a marring of other components, detracting from their normal functioning. Also, part of the dispersed toner may be deposited on the copy to degrade the image quality. In extreme cases, it may be driven out of the apparatus to cause a marring of the surrounding environment.
It will be noted that after the neutralizing step, there occurs no marring of the other side plate 2b by the toner since both the record medium and the toner are substantially neutralized.
In the prior art there has been no remedy to prevent a marring of the neutralizer by the toner powder. Usually, the neutralizer is allowed to be marred, and is sometimes cleaned.
There has been a proposal to provide a cover around the peripheral surface of the record medium in order to prevent a dispersion of the toner. However, a deposition of the toner on the internal surface of the cover as a result of its dispersion produces a gradual accumulation thereon, which must be removed at suitable intervals, in the same manner as the neutralizer is periodically cleaned.
It is an object of the invention, first, to provide an electronic copying apparatus capable of preventing a marring of a neutralizer and of effectively preventing a dispersion of a toner powder which may arise in connection with the operation of the neutralizer, and secondly, to provide an electronic copying apparatus capable of effectively preventing a dispersion of a toner powder before the transfer of a visible image.